DNA ligase is an important component of the machinery that all living cells use to maintain the integrity of their genetic material. This enzyme catalyzes the ligation of DNA strand breaks generated during various cellular metabolic events such as DNA replication and recombination. It also plays a crucial role in repairing DNA strand breaks generated in response to various environmental insults (1, 2). Higher eukaryotes have more than one type of DNA ligase. In fact, five distinct ligase activities have been reported in humans. Three human ligase genes LIG I, LIG II and LIG III have also been characterized (3). Ligase II is believed to be a proteolytic product of Ligase I. Ligase IIIa and IIIb are produced from LIG III by differential processing. In addition to being encoded by different genes, these ligases also exhibit differential substrate specificity and are localized in different cellular compartments. Therefore, it has been proposed that they are involved in different metabolic functions in eukaryotic organisms (1-4).
During the past three decades, DNA ligase genes have been cloned from a large number of organisms (1-4). The enzyme from higher eukaryotes is very similar to that from the viral or phage enzyme in its requirement for ATP. Bacterial enzymes require NAD as a co-factor. Nonetheless, all ligases share many common structural motifs characteristic of this family of proteins. Despite these similarities, there are important differences in the primary structure of DNA ligases from different species. This may account for the difficulty of cloning by homology of plant DNA ligases. The only plant DNA ligase cloned as of 1998 is from Arabidopsis thaliana. The maize DNA Ligase I of the present invention is the first ligase to be cloned from maize.
The need for the detailed characterization of maize DNA ligases for use in DNA integration is clear. Further, what is needed in the art is a means to improve transformation efficiency. The present invention provides these and other advantages.